Quantum wells are of considerable interest as components in the fabrication of high speed electronic devices. For example, a modulation-doped quantum well disposed between a pair of barrier layers can form a highly conductive layer referred to as a two-dimensional electron gas layer. Because the quantum well is free of dopant ions, electrons within the well exhibit much higher mobility than is found in conventional doped semiconductors. Such layers offer promise for use as interconnecting layers monolithically incorporated within semiconductors. Moreover, with close spacing between plural quantum layers, new types of devices can be designed using quantum mechanical tunneling between closely-spaced wells.
A difficulty impeding the development of such devices is the problem of making reliably electrical contact to individual quantum wells in a plurality of closely spaced wells. Because the wells are closely spaced--typically within a few hundred angstroms--conventional diffused electrical contacts to one quantum well also provide an appreciable conductive path to an adjacent well. Also because of the close spacing, efforts to isolate one well from another by conventional etching techniques require a control of etching depth not presently available on a reproducible basis. Accordingly, there is a need for apparatus for contacting individual ones of a plurality of closely-spaced quantum wells.